Disable late-combine for -O0 [PR115677]

late-combine relies on df, which for -O0 is only initialised late
(pass_df_initialize_no_opt, after split1).  Other df-based passes
cope with this by requiring optimize > 0, so this patch does the
same for late-combine.

gcc/
	PR rtl-optimization/115677
	* late-combine.cc (pass_late_combine::gate): New function.